Environmental Flow Patterns of Tropical Cyclone Genesis over the western North Pacific

Significant flow patterns of tropical cyclone genesis over the western North Pacific have been studied, which flow patterns are shear line, confluence region, monsoon gyre, easterly wave, and Rossby energy dispersion (Ritchie and Holland 1999). Yoshida and Ishikawa (2013) proposed an objective method to categorize tropical cyclone genesis into the flow patterns, and concluded major part of tropical cyclone genesis were occurred with shear line, confluence region, and easterly wave by a statistical analysis of 30 years. However, these characteristics are investigated based on the fact of tropical cyclone genesis. The flow patterns feature in non-genesis period has not been studied. The non-genesis period is days with no records for tropical cyclone genesis over the western North Pacific. Furthermore, it has not been revealed how significant the flow patterns are in genesis period. We extended the objective method of Yoshida and Ishikawa (2013) to all of the grid points of JRA-55 data, and got horizontal distributions of flow pattern indices over the western North Pacific for 38 years from 1979 to 2016. In this study, we focused on the frequent three patterns; shear line, confluence region, and easterly wave. A horizontal distribution of each flow pattern averaged about 38 years well represent that of tropical cyclone genesis over the western North Pacific. Shear line pattern locates on the east of Philippine islands, and confluence zone is found on the slightly south of the location of shear line pattern. Easterly wave pattern locates widely in longitudinal direction over the western North Pacific. The horizontal distributions and significance of flow patterns have a seasonal variability. These variations are similar to the variations of monsoon and intra-seasonal oscillation, and also similar to the distribution of genesis potential index. To examine how significant the flow patterns in genesis period, we compared the indices of flow patterns at the genesis point to the indices averaged about 38 years over the western North Pacific based on a best track data. It is clear that all three flow patterns are more significant in genesis period than non-genesis period. The time to be significant before the genesis is different among three patterns. Shear line and confluence zone becomes significant approximately 7 days before the genesis, while easterly wave is 3 days before the genesis. Comparing flow patterns with other environmental factors such as relative humidity and wind shear, we try to find the necessary condition for tropical cyclone genesis.